1. Field of the Invention
The present invention relates to two-dimension precision transfer equipment, three-dimension precision transfer equipment and coordinate measuring machine that transfer a work table or a column of measuring equipment and a machine tool with a high precision for positioning.
2. Description of Related Art
Precision transfer equipment requiring a high motion accuracy (e.g., a precision positioning system, precision measuring equipment and a machine tool) has been demanded to move an object to be moved (e.g., a work table and a main shaft column) with a high geometric accuracy of motion and, in addition, to accurately detect and control a position of the object in a linear moving direction.
A work table and a spindle column movable in two dimensions (i.e., X and Y directions) in a horizontal plane are frequently used.
For instance, in order to move a table in two directions of an X-axis and a Y-axis, two ball-screw mechanisms are provided to a base in a direction orthogonal to each other and the table movable in two directions (i.e., X and Y directions) is provided on an upper side of the two ball screw mechanisms, where the table is moved in each axial direction by each of the ball screw mechanisms (see Patent Literature 1: JP-A-2002-258950).
In addition to the ball screw mechanism, transmission mechanisms such as friction drive, belt drive, and rack-and-pinion are appropriately employed as a mechanism for driving in each axial direction.
For detecting and controlling a position of the object, it is desirable that the mechanism satisfies the Abbe's principle. In other words, a detecting axis for identifying the position of the object is desirably disposed to pass through a point of action for a measurement or processing performed by moving the object.
In the above Patent Literature 1, since the ball screw mechanisms are provided on the circumference of the table, an offset of a thrusting axis against a moving axis passing through the center of the object is large. In contrast, by layering a Y-axis drive mechanism on an X-axis drive mechanism to form a double-layer structure and providing the X-axis drive mechanism and the Y-axis drive mechanism immediately beneath the table, an offset between the moving axis and the thrusting axis is suppressible since the moving axis and the thrusting axis are drawn closer to each other (see Patent Literature 2: JPA-2006-114557).
Moreover, in some equipment, a driving force acts on a side of the table in order to reduce a vertical offset as described in Patent Literature 2 (see Patent Literature 3: JP-A-2006-205292).
In the equipment described in Patent Literature 3, a drive roller moves a rod forward and backward, the rod being connected to a side of the table to drive the table.
In Patent Literature 3, the rod and the table is connected by a fluid static-pressure joint. Especially, a supplying-discharging static-pressure bearing or a vacuum-balanced static-pressure air bearing can secure a strong rigidity in a compression direction and a tensile direction. With a drive mechanism using a static-pressure bearing having such a high rigidity, a geometric accuracy of motion of a highly precise linear moving table is improved.
The static-pressure joint disclosed in Patent Literature 3 further includes a thrust plate that is supported near the rod by a gimbal mechanism absorbing a motion error in a yawing direction and is supported near the table by a similar gimbal mechanism absorbing a motion error in a pitching direction. Through the joint, forces other than the force in the moving direction of the table are not transmitted.
With the use of such a drive mechanism of Patent Literature 3, the vertical offset as described above in Patent Literature 2 can be eliminated. In addition, since the X-axis drive mechanism and the Y-axis drive mechanism are provided on the respective sides of the table, the double-layer structure is not required, so that the total height of the equipment is reduced.
In each of the above-described transfer equipment, it is necessary not only to accurately drive the table but also to position the table with a high precision. For this reason, transfer equipment provided with a laser interferometer as a position detecting mechanism that detects a current position of the table with a high precision has been used (see Patent Literature 4: JP-A-2000-55617).
In the equipment of Patent Literature 4, a slider driving mechanism using a feed screw shaft is provided under the table. The feed screw shaft allows a horizontal movement of the table provided thereon. The laser interferometer is provided to a side of the table, whereby a current position of the table is detectable with a high precision.
In this arrangement, a laser path extending from a light source of the laser interferometer to an end of the table is surrounded by a stretchable tube with a bellows, in which an inner pressure of the tube is reduced to eliminate an influence from air turbulence in the laser path, thereby achieving further high precision.
With such a laser interferometer, it is possible to accurately detect and control a position of the movable body in a feed direction, in addition to a high geometric accuracy of motion of the drive mechanism and the movable body.
As described above, when the drive mechanism of Patent Literature 3 is used in the X-axis and the Y-axis, the moving axis in a predetermined moving direction which passes through the centroid of the table can be aligned with the thrusting axis of the driving force obtained by the drive mechanism and the total height of the equipment is reducible while satisfying the Abbe's principle.
In such highly precise transfer equipment, it is necessary to combine the laser interferometer of the Patent Literature 4 and the like with the transfer mechanism of each axis for a highly precise position-detection.
When a highly precise position-detection is conducted with use of the laser interferometer and the like, it is desirable that the detecting axis also satisfies the Abbe's principle and it is desirable that the laser path (i.e., the detecting axis) is aligned with the moving axis and the thrusting axis.
However, it is mechanically difficult to coaxially arrange the laser path of the laser interferometer of Patent Literature 4 and the driving axis by the rod and the static-pressure air joint of Patent Literature 3.
Particularly, in Patent Literature 4, in order to eliminate influences of a medium in the laser path, a bellows is disposed at a reduced inner pressure to surround the laser path. An end of the bellows needs to be hermetically connected to the table. Such a position detecting mechanism cannot be directly applied to the table moving in two directions of the X-axis and the Y-axis.
For instance, when the table is moved in the Y-axis direction, a connection portion of the bellows surrounding the laser path for position-detecting in the X-axis direction is displaced toward the side of the table, so that the laser path is bent or a mechanical connection of the bellows may become unmaintainable.